The radio, when in ATAS tuner mode supplies voltage through the coax to the antenna tuning circuits. When the ATAS sees a voltage of +12 VDC on the coax it will extend the antenna and when it sees a voltage of 8 VDC it will retract the antenna. The actual control of the antenna is built in the Yaesu radio and the software in it decides to send the required voltage to the antenna circuit.
I do not believe that there is any separate 12 volts sent to the antenna other than that from the radio through the coax. The radio, of course, has to be in the proper mode and working properly and it is designed to tune to better than a 2:1 SWR to protect the radio. When a signal combiner is used the VHF and UHF mode would be with the antenna fully retracted.
I? think that different radios may behave differently as to what happens if a good SWR is not found within so many cycles or so much time.?
